NOAA Scientists Link InternationalDroughts and Ocean Temperatures

Weather patterns could be consequence of greenhouse
gases

Droughts in the United States, Southern Europe and Southwest Asia in
1998-2002 were caused by the "perfect ocean for drought," according to a
study published in the magazine Science by researchers from the National
Oceanic and Atmospheric Administration (NOAA).

Cold sea surface temperatures in the eastern tropical Pacific and warm
sea surface temperatures in the western tropical Pacific and Indian Ocean
worked together to create an abnormal pattern in tropical rainfall,
according to a January 31 NOAA press release. Climate scientists Martin
Hoerling and Arun Kumar came to these conclusions after entering actual
sea surface temperatures in climate simulation computer programs.

The scientists said, "What is suggested by the atmospheric modeling
results of 1998-2002 is an increased risk for severe and synchronized
drying of the mid-latitudes in the future, if these oceanic conditions
continue to occur."

The unusual warming in the Pacific and Indian Oceans was attributed in
part to increased greenhouse gases, according to the press release. NOAA
officials say that further study should be conducted to learn more about
ocean temperatures and climate change.

Following is the text of the press release:

National Oceanic and Atmospheric Administration

January 31, 2003

NOAA Scientists Attribute Recent Droughts to Ocean
Influence

NOAA researchers studying the 1998-2002 droughts that spread across the
United States, Southern Europe and Southwest Asia, believe they were
linked by a common thread -- ocean conditions.

The findings are published in the Jan. 31 issue of Science. Lead author
Martin Hoerling, a scientist at the NOAA Climate Diagnostics Center in
Boulder, Colo., and colleague Arun Kumar, from the NOAA Climate Prediction
Center in Camp Springs, Md., say cold sea surface temperatures in the
eastern tropical Pacific and warm sea surface temperatures in the western
tropical Pacific and Indian Oceans worked together synergistically to
cause wide-spread drying in the mid latitudes. According to Hoerling, it
was the "perfect ocean for drought."

During 1998-2002, the prolonged below normal precipitation and above
normal temperatures caused the United States to experience drought in both
the Southwest and Western states and along the Eastern seaboard. These
droughts extended across southern Europe and Southwest Asia. "During the
four-year period, as little as 50 percent of the average rainfall fell in
these regions," said Hoerling. According to Hoerling, this was an abrupt
change for the United States from what had been ranked as the wettest
decade since at least the1890s.

Using climate simulations, the scientists assessed how the ocean
conditions over the four-year period influenced climate. "We used the true
monthly varying sea surface temperatures and then, using high-speed
computers, ran several climate models more than 50 times and averaged
their responses," Kumar said. "By running them multiple times, we could
identify the common, reproducible element of the atmosphere's sensitivity
to the ocean."

What the researchers found was that the tropical oceans had a
substantial effect on the atmosphere. "There were unprecedented warm sea
surface conditions in the western tropical Pacific, while at the same
time, we had three-plus consecutive years of cold La Niña conditions in
the eastern tropical Pacific," Hoerling said. "Usually, the La Niña
conditions would have cooled the whole ocean."

According to Hoerling, "The warmth in the west Pacific during 1998-2002
simply has no precedent in at least the past 150 years." The researchers
say that the combination of the warm and cold oceans shifted the tropical
rainfall patterns into the far west equatorial Pacific.

What caused the remarkable conditions that occurred in the 1998-2002
period? The researchers say that while the cold sea surface temperatures
were unusual, they were not unprecedented, but the warmth of the tropical
Indian Ocean and the west Pacific Ocean was unsurpassed during the 20th
Century. "Climate attribution studies find that this warming (roughly 1
degree Celsius since 1950) is beyond that expected of natural variability
and is partly due to the ocean's response to increased greenhouse gases,"
they said.

The scientists added, "What is suggested by the atmospheric modeling
results of 1998-2002 is an increased risk for severe and synchronized
drying of the mid latitudes in the future, if these oceanic conditions
continue to occur."

Randall Dole, director of the NOAA Climate Diagnostics Center, says
"The study provides compelling evidence for the crucial role that the
tropical oceans played in producing widespread severe and sustained
drought over the period 1998-2002."

Dole says that while the study's primary focus was not to analyze the
causes of the warming of the tropical oceans, the study does suggest that
these droughts may be partly related to climate change and that further
work needs to be done to completely understand the unprecedented warming
of the western Pacific.

NOAA is dedicated to enhancing economic security and national safety
through the prediction and research of weather and climate-related events
and providing environmental stewardship of the nation’s coastal and marine
resources.

NOAA is part of the Department of Commerce.

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